If you ever accidentally switch the site language for your Duolingo...
Follow the steps below provided by this link: https://www.duolingo.com/comment/3318634
In my case, I had accidentally switched to Vietnamese, which I only partially understand. I speak English and I am learning French. I don't even know how I switched it. If you change to Vietnamese, go to "hoc ngon ngu" for step #3 (follow the link first then do this). Then press save changes, which will read, "luu thay doi." For French, it will say "sauvegarder les changements" or "sauver les changements," or something like that.
Clicking the flag on the left side of the home page directly takes you to the link. Then you just need to select the language from the drop down and save the change :)
For the save option, just look for the green highlighted option after the language change.